The present invention relates to a semiconductor optical amplifying element for use in fiber optic communication.
Since a semiconductor optical amplifier has a high internal gain and a small size and is highly reliable, much study is being given it for use not only as a direct optical amplifying element of a nonregenerative repeating system in an intensity modulation-direct detection (IM-DD) system for fiber optic transmission and in a coherent transmission system but also as a pre-amplifier of a photodetector and a booster amplifier in an optical circuit. From the viewpoint of enhancement of system characteristics, the realization of a semiconductor optical amplifying element is highly desired which is excellent in terms of high saturation output, high gain, low noise, wide bandwidth and low polarization dependence. In this case, incident light, even if initially linearly polarized, generally becomes elliptically polarized after its long distance propagation through a fiber, and besides, the elliptically polarized wave fluctuates with the lapse of time. On this account, the intensity of output light amplified by the semiconductor optical amplifying element undergoes irregular variations, and hence no fixed gain can be obtained.